1. Field of the Invention
The present invention relates to a control device of an internal combustion engine, in particular, to a control device of a premixed charge compression ignition (PCCI) internal combustion engine capable of stable operation over a wide operating range.
2. Related Art
Conventionally, a diffusion combustion system used for a diesel engine compresses air drawn into a combustion chamber, and then injects fuel into this compressed air to cause self-ignition, thereby causing combustion. Such a diesel engine has thermal efficiency superior to that of a gasoline engine. However, since there locally exists an excessive fuel concentration region and a high temperature region due to non-uniformly atomized fuel distribution, the amount of NOx and particulate matter (herein after referred to as “PM”) emissions is high, which results in the load placed on an exhaust-gas aftertreatment system of exhaust being large.
In view of this problem, recently, the premixed charge compression ignition has received attention with the aim of considerably reducing the emission amounts of NOx and PM while maintaining fuel efficiency. With the premixed charge compression ignition, in contrast to the above-mentioned diffuse combustion, compression ignition is caused of a premixed air-fuel mixture that is diluted during a long ignition retarding period.
FIG. 9 is a diagram illustrating regions of PM and NOx generation in relation to a local temperature and a local equivalence ratio. As shown in FIG. 9, PM is generated due to oxygen deprivation in a region where the local equivalence ratio is high (fuel is excessively concentrated), and NOx is generated in a region where the local equivalence ratio is low and the local temperature is high.
As shown by the dashed line 9a, in the conventional diffusion combustion, the combustion occurs over both regions of PM and NOx generation by compression ignition of non-uniformly atomized fuel. On the other hand, as shown by the solid line 9b, in the premixed charge compression ignition combustion, by compression ignition of the diluted premixed air-fuel mixture, regions of excessive fuel concentration and regions of high temperature are few compared with conventional diffusion combustion, and the generated amount of PM and the NOx can be reduced.
Various studies have been made recently on such a premixed charge compression ignition engines. For example, in Patent Documents 1 and 2, for the purpose of further improving fuel efficiency and exhaust emissions, it is proposed that an engine is provided with a mixing device that mixes a first fuel with intake air to supply a premixed air-fuel mixture, and an injector that injects a second fuel directly into a combustion chamber. In this engine, the second fuel is injected directly into the combustion chamber after the premixed air-fuel mixture has ignited in the combustion chamber. Furthermore, in Patent Documents 1 and 2, combinations of fuels such as natural gas, gasoline, light oil, naphtha, and propane are exemplified as the first fuel and the second fuel.    [Patent Document 1] Japanese Translations of PCT International Publication, Publication No. 2003-532828    [Patent Document 2] Japanese Translations of PCT International Publication, Publication No. 2003-532829